A major goal of restorative dentistry is the replacement of carious lesions with a material which most closely resembles the lost tissue, aesthetically and functionally. While Current resin composites posses desirable properties including aesthetics, wear resistance, mechanical strength and coefficient of thermal expansion approaching that of tooth structure, they undergo sufficient cure shrinkage to create interfacial stresses of such magnitude that marginal seals cannot be reliably maintained in large posterior restorations despite the tremendous gains made in dentin bonding in recent years. Our goal is to develop new resin composites which possess the mechanical and aesthetic properties of current resins, while greatly simplifying the task of bonding to eliminate marginal leakage. We base our goal on the observation that nematic liquid crystal (LC) monomers can polymerize to isotropic polymers. Our hypothesis is that appropriately selected liquid crystal monomers can be synthesized for use as matrix resins in dental restorative materials which possess- the desirable properties of current matrix resins while undergoing zero polymerization shrinkage. This hypothesis is based on the volume expansion which accompanies transformation from an ordered LC nematic structure to an isotropic solid. Specific Aim 1 will synthesize several candidate diacrylate and dimethacrylate monomers and characterize them as to mesophase transformation temperatures and polymerization shrinkages. Critical to this aim will be the development of monomers which are nematic LC at room temperature when pure, or when solvated by conventional comonomers, or when highly filled with nanofillers. Specific Aim 2 will develop model resin Composites based on the best LC monomer(s) from specific aim 1 and conventional silanized small particle barium glass fillers. Specific Aim 3 will develop model resin composites based on the best LC monomer(s)from specific aim 1 and hybrid tantalum oxide nanofillers developed in Project 1, Nanofiller Development. The resin composites developed under specific aims 2 and 3 will be completely characterized for critical performance properties including in vitro and in vivo.